The corbel is defined as the brick or block conical portion of a manhole that tapers upward and inward from the sewage line to the frame diameter of the chimney that connects to the manhole. Corbels serve the purpose of providing access to the sewer system or drain for easy cleaning and servicing as well as sewage aeration.
Trenchlesspedia Explains Corbel
Corbels are made to project upward and taper inward to reduce the size of the manhole opening required on the ground surface. The maximum allowance for corbelling is 2” from the brick underneath. Corbels are installed in areas where there is constant flow of vehicular traffic and pedestrians. It allows for aeration of sewage, and run off from rain and surface water under gravity or when the pressure is low in addition to granting access for cleaning purposes.